Full color organic electroluminescent (EL) devices, also known as organic light-emitting diode (OLED) devices, have been demonstrated as a new type of flat panel display. OLED devices are attractive because of their low driving voltage, high luminance, wide-angle viewing and capability for full-color flat emission displays. In its simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic EL medium sandwiched between these electrodes to support charge recombination that yields emission of light. An example of an organic EL device is described in commonly-assigned U.S. Pat. No. 4,356,429. Other examples have been described by Tang et al. in U.S. Pat. Nos. 4,769,292 and 4,885,211. In order to construct a pixilated display device that is useful, for example, as a television, computer monitor, cell phone display or digital camera display, individual organic EL elements can be arranged as an array of pixels in a matrix pattern. This matrix of pixels can be electrically driven using either a simple passive matrix or an active matrix driving scheme. In a passive matrix, the organic EL layers are sandwiched between two sets of orthogonal electrodes arranged in rows and columns. An example of a passive-matrix-driven organic EL device is disclosed in commonly-assigned U.S. Pat. No. 5,276,380. In an active matrix configuration, each pixel is driven by multiple circuit elements such as transistors, capacitors, and signal lines. Examples of such active matrix organic EL devices are provided in U.S. Pat. No. 5,550,066 (commonly-assigned); U.S. Pat. Nos. 6,281,634; and 6,456,013.
One way of improving the efficiency of an OLED device is the use of a microcavity structure to enhance emission at a specific wavelength. A reflector and a semitransparent reflector function, with the layers between them, to form a microcavity, which can be adjusted in thickness and refractive index to resonate at a desired wavelength. Examples of microcavity structures are shown in U.S. Pat. Nos. 6,406,801; 5,780,174 A1; and JP 11288786. A broadband-emitting OLED material can be used, and by varying the length of the cavity, different colored emission can be achieved. However, the resonant wavelength of a microcavity is also a function of angle and thus the color emitted by a microcavity OLED tends to have an undesirable dependence on angle, generally shifting toward the shorter wavelengths as the viewing angle shifts from normal to more oblique angles. The emission intensity of microcavity devices is also characteristically directional, and falls off fairly rapidly with viewing angle. (See, for example, N. Takada, T. Tsutsui, and S. Saito Appl. Phys. Lett. 63(15) 2032 (1993) “Control of emission characteristics in organic thin film electroluminescent diodes using an optical microcavity structure”.) There exists a need for an efficient OLED with reduced image dependence on viewing angle.